Did some more testing by removing the doctype from the header.php file… I know now it’s not a doctype problem, because the problem now jumped to the next line where the first 25 characters look as weird as you can see in the screenshot above.
Edit:
Maybe any of these two warning/error ring a bell with someone.
Byte-Order Mark found in UTF-8 File.
The Unicode Byte-Order Mark (BOM) in UTF-8 encoded files is known to cause problems for some text editors and older browsers. You may want to consider avoiding its use until it is better supported.
Validation Output: 1 Error
Line 1, Column 1: character “?” not allowed in prolog
??<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “https://www.w3…
The validation error only occures on single-..php and taxonomy.php files, even when I only put the get_header() in it. The BOM warning appears on every page.
